But the news of Ms Ardern's promotion was quick to hit the streets of her electorate, Mt Albert.

People on the streets were happy, telling Newshub they "really like her".

"She's a very lovely lady, I've met her," one person said, while another said: "Good move, I think they needed to do something."

One woman said it's a win for feminism having a female leader.

While for Prime Minister Bill English, the message that Ms Ardern is now his number one rival came through loud and clear.

"This is someone who in a matter of months could be Prime Minister and in the context of the current election campaign, we need to lift our levels of support to win an election with Jacinda Ardern as leader of the opposition," he said.

And political commentators from both sides of the political spectrum are telling Mr English to watch his back.

"National keeps telling itself not to be complacent I think it should be doing a bit more than telling itself not to be complacent," Matthew Hooton says.

On the other end of the spectrum, Chris Trotter says: "They're going to be going damn, damn, damn, because I think this is a bit of a gamechanger."

ACT leader David Seymour was quick to discount her as a potential Prime Minister.

"I just don't think it's credible to have a 37-year-old Prime Minister in coalition with Metiria Turei and Winston Peters," he said.

For the record, Mr Seymour is 34 - although he points out it's not hypocritical because he's not trying for Prime Minister.

Ms Ardern's former Auckland Central opponent Nikki Kaye was much was more complimentary, offering her congratulations.

Labour's potential coalition partners were as gracious.

"I hold her very high regard and I'm very pleased that she has this new role," Green Party co-leader Metiria Turei said.

Ms Ardern is already tempting Winston Peters with a cheeky post-election tipple, telling him she likes single malt whiskey.

Though he wasn't having it, refusing to speak to media about the leadership today.

While Ms Ardern's promotion was announced with much excitement, it leaves Labour with a little problem - their former leader's face is still all over the billboards.

But the party will order new ones this week, some with the new pair and some just with Ms Ardern. They'll be up as soon as they're off the printers.

Party Secretary Andrew Kirton says they're not sure yet whether they'll just cover over Mr Little with stickers or do a reprint, and he's not sure of the cost.

"We'll work through that, I'll get the old calculator out," he said.

But it'll be a bill worth footing if their new leader can bring them a victory.